The present invention is related to a method for re-generating supercritical extractant, and in particular a method for re-generating supercritical CO2.
Because supercritical carbon dioxide (SCCO2) possesses several unique characteristics and physicochemical properties, such as being nontoxic, nonflammable, inexpensive, and having the extraction power tuned by temperature and pressure, and a higher mass transfer rates than conventional organic solvents, it has been used as a green solvent for decaffeination and extraction of flavors processes. Due to that the solubility of a solute in supercritical carbon dioxide is relatively low and increases with pressure, the operation pressure required is generally high and a large quantity of carbon dioxide is needed. To make an extraction process more economic, regeneration of supercritical carbon dioxide for cycling use is essential.
Several regeneration techniques including reduction of pressure, adsorption, absorption, and membrane have been investigated. Though the simplest means for regeneration of supercritical carbon dioxide is to reduce pressure to causing a phase separation, the energy required to recompress the expanded gas is usually high. While an isobaric and/or isothermal operation to remove solute from a SCCO2 mixture may be possible with adsorption or absorption, a proper adsorbent or absorption solvent is not easy to obtain and solute cannot be completely removed in general. Besides a further treatment of adsorbent or absorption solvent is needed. When the size sieving is considered as the major mechanism in a membrane separation process, a membrane with a desired pore size distribution is definitely needed. Generally the difference in molecular sizes between carbon dioxide and the solute loaded is not in a large extent, a proper selection of pore size therefore needs to pay attention to. It was known that the molecular sieving mechanism may be applied to obtain a completely pure SCCO2 if a membrane with the pore size sufficiently close to that of CO2 can be synthesized. However, the use of this membrane inevitably causes the permeation flux of SCCO2 decrease due to the small pore size thereof.
The present invention discloses a method for re-generating supercritical extractant from a mixture containing said supercritical extractant and a solute in said supercritical extractant, which comprises contacting said mixture with a molecular sieve membrane at a temperature and a pressure in a critical region of said extractant and near a critical point of said extractant so that a permeate rich in said extractant and a retentate having a enriched concentration of said solute in said extractant are generated, wherein said molecular sieve membrane has a pore size significantly larger than a size of said extractant provided that said pore size is less than and close to sizes of clusters formed of said solute and said extractant at said temperature and said pressure.
Preferably, said extractant is CO2.
Preferably, said solute is selected from spices, hops, pesticides or caffeine, and more preferably caffeine.
Preferably, said molecular sieve membrane has a pore size about 1-5 nm.
Preferably, said temperature is of 30-50xc2x0 C. and said pressure is of 7.0-9.5 MPa.
Preferably, a pressure drop across said molecular sieve membrane during said contacting is of about 0.1-0.8 MPa.
Preferably, a solute rejection defined as (Cfeedxe2x88x92Cpermeate)/Cfeed is greater than 0.8, and more preferably, greater than 0.9, wherein said Cfeed is a concentration of said solute in said mixture and said Cpermeate is a concentration of said solute in said permeate.